Resistance heating element for extreme temperatures

ABSTRACT

An electrical resistance heating element operable at extremely high temperature, up 2300° C. when used in a vacuum or in a reducing atmosphere, and up to bout 1200° C. when used in an oxidizing atmosphere. The element is formed substantially from titanium silicon carbide (Ti 3 SiC 2 ), which is readily workable to enable it to be produced in different forms. It also has a higher mechanical strength than that of graphite heating elements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel electrical resistance heatingelement for extremely high temperatures.

The heating element is intended for use in connection with heattreatment and sintering in inert and reducing atmospheres, in oxidizingatmospheres, and in a vacuum environment, up to extremely hightemperatures, such as 2300° C.

2. Description of the Related Art

Resistance heating elements of the present type are manufactured byApplicant (Sandvik A B, of Sandviken, Sweden), such as resistanceelements based on NiCr, FeCrAI, SiC, and MoSi₂. These materials are usedin a plurality of atmospheres and at mutually different temperatures. Acommon feature of these materials is that their usefulness at hightemperatures, i.e., temperatures above 1300° C., is limited in reducingenvironments and under vacuum conditions.

Heating elements comprised primarily of Mo, W, Ta (tantalum),. andgraphite are used for temperatures up to above 2000° C. Each of thesematerials has drawbacks and limitations, a common feature being thatsaid materials oxidize readily when subjected to temperatures above400-500° C., thereby requiring particular attention to be paid tofurnace design and furnace operation. The high specific weight of Ta,Mo, and W also necessitates limitations with respect to mechanicalconstruction. Further, material costs for refractory metals are high inthemselves.

SUMMARY OF THE INVENTION

The present invention relates to resistance heating elements comprisedof a novel material.

The present invention thus relates to an electrical resistance elementfor heating purposes, which element is comprised substantially oftitanium silicon carbide (Ti₃SiC₂).

According to the invention, the resistance element is substantiallycomprised of titanium silicon carbide (Ti₃SiC₂).

The element may consist of almost 100 weight percent Ti₃SiC₂.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a preferred embodiment, said resistance element consists atleast of 85 weight % titanium silicon carbide.

According to a preferred embodiment, the material contains up to 15weight % of one or more of the compounds TiC, TiSi₂, Ti₅Si₃, or SiC, andthus at least 85 weight % titanium silicon carbide.,

In its pure form, titanium silicon carbide is a relatively new ceramicmaterial that possesses very good mechanical and thermal properties,among other things.

In texture, the material is of a so-called nanolaminate type, whichcontributes to very high impact durability and high thermal shockresistance. The material can be readily worked and, for instance, can becut with a saw, which is unique in respect of ceramic material.

The material is relatively light, having a density of 4.5 g/cm³. Thethermal conductivity is relatively constant at about 35 W/mK and thematerial is also a good conductor of electricity.

The material can be deformed plastically at high temperatures. Theshrink properties of the material exceed those of so-called superalloysthat are used up to temperatures of 1100° C., this temperature by nomeans constituting an upper temperature limit for Ti₃SiC₂.

One highly significant property is that the oxidation properties ofTi₃SiC₂ surpass the oxidation properties of Mo, W, Ta (tantalum), andgraphite.

The material Ti₃SiC₂ exists up to a temperature of 2330° C. It breaksdown into TiC_(x) and Si at this temperature, thus placing in practice atemperature limit of 2300° C. when using said material.

The properties of a phase pure material of Ti₃SiC₂ were relativelyunknown until recently, due to the difficulty of producing such amaterial.

Measurements carried out on the electrical resistance of denselysintered wire, produced from Ti₃SiC₂ powder in an argon atmosphere attemperatures ranging from 20-1600° C., showed that the resistance of thematerial within this range varied linearly from about 0.28 ohm*mm²/m toabout 1.43 ohm*mm²/m. This lies on the level of MoSi₂ heating elementsat room temperature, but is about 55% lower than the level for MoSi₂heating elements at 1600° C. It has been found that the stability of thematerial is favored by decreasing partial pressures of oxygen, whichshows that the material can be used successfully in vacuum furnacesunder high vacuum, in which low degassing rates from the heatingelements is essential.

It has also been found, surprisingly, that the material remainsunaffected when exposed to dry hydrogen gas up to a temperature of atleast 1800° C. This temperature is the maximum temperature at which thetest was carried out, although it is likely that the material willremain stable in dry hydrogen gas up to still higher temperatures.

Titanium silicon carbide has also been found to have very goodproperties in oxidizing atmospheres, such as air, up to temperatures of1100-1200° C. In this case, the surface oxide of the element iscomprised of TiO₂. Oxides in the form of SiO₂—TiO₂ are present inwardlyof the surface oxide. However, the properties of the titanium siliconcarbide in oxidizing atmospheres are surpassed at these temperatures andhigher temperatures by alloys of the type FeCrAl and MoSi₂. Thesematerials are produced by applicant in a number of forms, retailed underthe trademarks KANTHAL and KANTHAL SUPER. Ti₃SiC₂ does not thereforeprovide a better alternative to these alloys in an oxidizingenvironment.

Titanium silicon carbide is also resistant over relatively long periodsof time during oxidation up to 1700° C. in air, in comparison withelements comprised of Mo, W, Ta (tantalum), and graphite. This propertymeans that the material will not be consumed over a short time period,should it not be possible to maintain an inert or reducing atmosphere inthe furnace for some reason or another.

By way of summary, it can be said that the inventive resistor element isparticularly suited for use in a vacuum or in an inert or a reducingatmosphere up to a temperature of 2300° C., and in oxidizing atmospheresup to a temperature of about 1200° C.

Furnaces equipped with Ti₃SiC₂ heating elements need not necessarilyhave a gas-tight enclosure that protects against oxidation, since thematerial is considerably more resistant to oxidation than Ta, Mo, W, andgraphite. This facilitates furnace construction.

The mechanical strength of Ti₃SiC₂ is much greater than graphite heatingelements, which are also used in a vacuum environment and in an inertand reducing atmosphere.

Another significant advantage with Ti₃SiC₂ is that relatively thinheating elements can be produced. This enables the electrical resistanceto be kept high, resulting in a not excessively low voltage across theelement. The electrical resistance of Ti₃SiC₂ at room temperature isabout six times higher than Mo and W, thereby reducing the cost ofrequisite control and regulating equipment.

Heating elements comprised of Ti₃SiC₂ can be made in many differentforms, such as in the form of rods, tubes, crucibles (with or withoutelectrode patterns), thin layers that include an electrode pattern (ofthe “electric oven plate” or surface heater type), slabs, bands. Forexample, elements can be produced in rod form and given a U-shape andprovided with coarser connectors, in the same way as in the case ofelements of the MoSi₂ - type. Thin layers provided with electrodepatterns constitute an alternative to the conventional filament-typeheating element design.

The good workability of Ti₃SiC₂ enables complicated geometries to beproduced at low cost the aid of simple means.

In addition to its use in heating elements, Ti₃SiC₂ can also be used infurnaces for different purposes, such as product-carrying material, heatshields, furnace rollers and furnace beams, mufflers, hangers, belts andchains, lead-throughs, and also as furnace linings.

1. An electrical resistance heating element for heating purposes,comprising a material that is substantially Ti₃SiC₂.
 2. An electricalresistance heating element according to claim 1, wherein said materialincludes at least 85 weight % Ti₃SiC₂.
 3. An electrical resistanceheating element according to claim 1, in which the material includes upto 15 weight % of a compound selected from the group consisting of TiC,TiSi₂, Ti₅Si₃, and SiC.
 4. An electrical resistance heating element forheating purposes consisting essentially of Ti₃SiC₂.